Visible light-induced biocidal activities and mechanistic study of neutral porphyrin derivatives against S. aureus and E. coli.

Positive charged porphyrins have long been regarded as effective biocidal agents, however neutral porphyrins have rarely been studied in their ability photoinactivating microbials, and the structure-activity relationship such as correlation of electronic effect and biocidal activity of porphyrins still remains unclear. Herein, four neutral porphyrins with various electronic effects were selected to undergo light-induced biocidal processes. It turned out that the TPPOH and TPPNH2 with electron-donating groups NH2 and OH, respectively, exhibited much more powerful light-induced biocidal activities against E. coli and S. aureus than TPP and TPPNO2 with electron-withdrawing group NO2. This phenomenon suggested that neutral porphyrins may be treated as a new class of biocidal agents and functional groups with various electronic effects on porphyrins can dramatically affect porphyrins' light-induced biocidal activities. Mechanistic studies demonstrate that despite a better light-induced antibacterial ability of TPPOH, its singlet oxygen generation efficacy is a little lower than that of TPPNH2, together with charge characteristics and lipophilicity, it is clear that (1) the oxidative species singlet oxygen and ROS played the key role in the photo-activated antimicrobial processes of porphyrins, and (2) higher singlet oxygen or ROS yields of TPPOH and TPPNH2 may originate from their structural characteristics, namely electron-donating groups OH or NH2, and (3) a synergistic effect of all other factors including the electrostatic and hydrophobic effects must involve in the process and cooperatively determine their biocidal activities.